In order to increase the computational speed of processors and the storage volume of memory elements and to reduce the costs for the components, chips which have increasingly small structures and hence an increasingly high density of components are being developed in the semiconductor industry. A particular challenge is the reduction of the minimum structure size. To date, these requirements have been met in optical lithography by changing over to increasingly short wavelengths. At a structure size of from 100 to 70 nm, however, processes known to date which use wavelengths down to 193 nm come up against the limit of their resolution. The development of novel processes which use the shorter exposure wavelengths is therefore necessary. Optical lithography has particularly good prospects of being capable of being used industrially, a radiation having a wavelength of 157 nm being used for the exposure, since in this case the chip manufacturers can continue to utilize their extensive know-how of optical lithography. A substantial difficulty with the use of an exposure radiation having a wavelength of 157 nm is the insufficient transparency of the materials used to date as photoresists. For industrial use, the base polymer in these high-resolution resists must have as high a transparency as possible, and various fluorine-containing photoresists have therefore been proposed for use in 153 nm technology.
In the exposure of the photoresist, however, troublesome reflections or scattering effects at the coated interfaces frequently occur, so that the coating which is arranged under the photoresist is necessary to reduce these reflections or scattering. A so-called bottom antireflective coating (“BARC”) which, in addition to avoiding reflections and scattering effects, also permits simultaneous planarization of already structured surfaces when it is applied by spin coating, is therefore applied between the substrate and the photoresist.
For 193 nm lithography, various BARCs have been developed that are formed either from inorganic materials, such as, for example, TiN, SiON or carbon, or from organic polymers. The present invention relates to a bottom antireflective coating which is composed of organic polymers.
Furthermore, the substrate reflections cannot be completely controlled with the 193 nm BARCs since, as a rule, the k value of the extinction coefficient at a wavelength of 157 nm is small. Another difficulty is that the photons at 157 nm have a higher energy than the photons at 193 nm, so that the polymer bonds of the photoresist may be cleaved, which may lead to the development of the low molecular weight polymer fragments and hence to the formation of gaseous products.
The organic BARC must also meet a number of further requirements. A BARC for 153 nm lithography must have not only suitable optical properties, such as, for example, refractive index (n), extinction coefficient (k), but also good adhesion properties with respect to silicon on the one hand and with respect to the photoresist on the other hand. The BARC must also have a higher etching rate compared with the photoresist in dry etching processes, and minimize formation of gaseous products. A further requirement for the BARC is that the BARC must not be softened or even dissolved by the solvent used for the photoresist, and that the edges structured after the etching must have a right angle, or that no “footing” forms.
Suitable properties which a BARC should have for 157 nm lithography are described in Claypool et al., Advances in Resist Technology and Processing, Proceedings of SPIE, Vol. 4690, 2002, pages 1065-1073, the disclosure of which is incorporated herein by reference in its entirety. In particular, the BARC must have, at 157 nm, a k value of from 0 to 0.70, a refractive index of from 1 to 2 and a thickness of less than 200 nm.
An exemplary BARC for use in 157 nm lithography is described in Irie et al., J. Photopolym. Sci. Technologie, Vol. 16, No. 4, 2003, pages 565-572, the disclosure of which is incorporated herein by reference in its entirety. The BAR disclosed in this reference consists of an acrylic polymer, novolak or heterocyclic polymer, where the polymers contain a side chain with Br or I atoms that absorb strongly at 157 nm. The content of halogen atoms is extremely high and is from 40 to 50% by weight of the polymer. The exact composition of the polymer is, however, not described.